1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations and, more specifically, to superabrasive cutters suitable for use on such bits, particularly of the so-called fixed cutter or xe2x80x9cdragxe2x80x9d bit variety.
2. State of the Art
Fixed-cutter, or drag, bits have been employed in subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. Polycrystalline diamond compact (PDC) cutters comprised of a diamond table formed under ultra-high temperature, ultra-high pressure conditions onto a substrate, typically of cemented tungsten carbide (WC), were introduced into the market about twenty-five years ago. PDC cutters, with their diamond tables providing a relatively large, two-dimensional cutting face (usually of circular, semi-circular or tombstone shape, although other configurations are known) have provided drag bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not previously possible with the smaller natural diamond and polyhedral, unbacked synthetic diamonds previously employed in drag bits. The PDC cutters have, with various bit designs, achieved outstanding advances in drilling efficiency and rate of penetration (ROP) when employed in soft to medium hardness formations, and the larger cutting face dimensions and attendant greater extension or xe2x80x9cexposurexe2x80x9d above the bit crown have afforded the opportunity for greatly improved bit hydraulics for cutter lubrication and cooling and formation debris removal. The same type and magnitude of advances in drag bit design in terms of cutter robustness and longevity, particularly for drilling rock of medium to high compressive strength, have unfortunately, not been realized to a desired degree.
State of the art substrate-supported PDC cutters have demonstrated a notable susceptibility to spalling and fracture of the PDC diamond layer or table when subjected to the severe downhole environment attendant to drilling rock formations of moderate to high compressive strength, on the order of nine to twelve kpsi and above, unconfined. Engagement of such formations by the PDC cutters occurs under high weight on bit (WOB) required to drill such formations and high impact loads from torque oscillations. These conditions are aggravated by the periodic high loading and unloading of the cutting elements as the bit impacts against the unforgiving surface of the formation due to drill string flex, bounce and oscillation, bit whirl and wobble, and varying WOB. High compressive strength rock, or softer formations containing stringers of a different, higher compressive strength, thus may produce severe damage to, if not catastrophic failure of, the PDC diamond tables. Furthermore, bits are subjected to severe vibration and shock loads induced by movement during drilling between rock of different compressive strengths, for example, when the bit abruptly encounters a moderately hard strata after drilling through soft rock.
Severe damage to even a single cutter on a PDC cutter-laden bit crown can drastically reduce efficiency of the bit. If there is more than one cutter at the radial location of a failed cutter, failure of one may soon cause the others to be overstressed and to fail in a xe2x80x9cdominoxe2x80x9d effect. As even relatively minor damage may quickly accelerate the degradation of the PDC cutters, many drilling operators lack confidence in PDC cutter drag bits for hard and stringer-laden formations.
It has been recognized in the art that the sharp, typically 90xc2x0 edge of an unworn, conventional PDC cutter element is usually susceptible to damage during its initial engagement with a hard formation, particularly if that engagement includes even a relatively minor impact. It has also been recognized that pre-beveling or pre-chamfering of the PDC diamond table cutting edge provides some degree of protection against cutter damage during initial engagement with the formation, the PDC cutters being demonstrably less susceptible to damage after a wear flat has begun to form on the diamond table and substrate.
U.S. Pat. Nos. Re 32,036, 4,109,737, 4,987,800, and 5,016,718 disclose and illustrate beveled or chamfered PDC cutting elements as well as alternative modifications such as rounded (radiused) edges and perforated edges which fracture into a chamfer-like configuration. U.S. Pat. No. 5,437,343, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates a multiple-chamfer PDC diamond table edge configuration which under some conditions exhibits even greater resistance to impact-induced cutter damage. U.S. Pat. No. 5,706,906, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates PDC cutters employing a relatively thick diamond table and a very large chamfer, or so-called xe2x80x9crake land,xe2x80x9d at the diamond table periphery.
However, even with the PDC cutting element edge configuration modifications employed in the art, cutter damage remains an all-too-frequent occurrence when drilling formations of moderate to high compressive strengths and stringer-laden formations.
Another approach to enhancing the robustness of PDC cutters has been the use of variously-configured boundaries or xe2x80x9cinterfacesxe2x80x9d between the diamond table and the supporting substrate. Some of these interface configurations are intended to enhance the bond between the diamond table and the substrate, while others are intended to modify the types, concentrations and locations of stresses (compressive, tensile) resident in the diamond tables and substrates after the cutter is formed in an ultra-high pressure, ultra-high temperature process, as is known in the art. Still other interface configurations are dictated by other objectives, such as particularly desired cutting face topographies. Additional interface configurations are employed in so-called cutter xe2x80x9cinsertsxe2x80x9d used on the rotatable cones of rock bits. Examples of a variety of interface configurations may be found, by way of example only, in U.S. Pat. Nos. 4,109,737, 4,858,707, 5,351,772, 5,460,233, 5,484,330, 5,486,137, 5,494,477, 5,499,688, 5,544,713, 5,605,199, 5,647,449, 5,706,906 and 5,711,702.
While cutting faces have been designed with features to accommodate and direct forces imposed on PDC cutters, see, for example, above-referenced U.S. Pat. No. 5,706,906, state-of-the-art PDC cutters have, to date, failed to adequately accommodate such forces at the diamond table-to-substrate interface, resulting in a susceptibility to spalling and fracture in that area. While the magnitude and direction of such forces might, at first impression, seem to be predictable and easily accommodated, based upon cutter back rake and WOB, such is not the case, due to the variables encountered during a drilling operation, previously noted herein. Therefore, it would be desirable to provide a PDC cutter having a diamond table/substrate end face interface able to accommodate the wide swings in both magnitude and direction of forces encountered by PDC cutters during actual drilling operations, particularly in drilling formations of medium-to-high compressive strength rock, or containing stringers of such rock, while at the same time providing a superior mechanical connection between the diamond and substrate and sufficient diamond volume across the cutting face for drilling an extended borehole interval.
The present invention addresses the requirements stated above, and includes PDC cutters having an enhanced diamond table-to-substrate interface, as well as drill bits so equipped.
The cutters of the present invention, while having demonstrated utility in the context of PDC cutters, encompass any cutters employing superabrasive material of other types, such as thermally stable PDC material and cubic boron nitride compacts. The inventive cutters may be said to comprise, in broad terms, cutters having a superabrasive table formed on and mounted to a supporting substrate. Again, while a cemented WC substrate may be usually employed, substrates employing other materials in addition to, or in lieu of, WC may be employed in the invention.
The inventive cutter comprises a table comprising a volume of superabrasive material and exhibiting a two-dimensional, circular cutting face mounted to an end face of a cylindrical substrate. An interface between the end face of the substrate and the volume of superabrasive material includes at least one annular surface of substrate material which is defined, in cross-section taken across and parallel to the longitudinal axis of the cutter, by an arc. The annular surface is preferably a spherical, or spheroidal, surface of revolution about the longitudinal axis of the cutter, or a portion of a toroid transverse to and centered on the longitudinal axis. If a spherical surface of revolution is employed, the center point thereof lies coincident with the longitudinal axis or centerline of the cutter. The surface of revolution may or may not extend at its outer periphery to the side of the substrate and is bounded at its inner periphery by another surface of revolution. The center of the substrate end face lying within the annular surface of revolution may exhibit a variety of topographic configurations. The superabrasive table formed over the substrate end face conforms thereto along the interface, while the exterior surface of the table may be provided with features such as chamfers as are conventional and known in the art.
The annular surface of the substrate end face, by virtue of its arcuate cross-sectional configuration, provides an interface designed to address multi-directional resultant loading of the cutting edge at the periphery of the cutting face of the superabrasive table. In general, resultant loads at the cutting edge are directed at an angle with respect to the longitudinal axis or centerline of the cutter which varies between about 20xc2x0 and about 70xc2x0. The arcuate surface is designed so that a normal vector to the substrate material will lie parallel to, and opposing, the force vector loading the cutting edge of the cutter. Stated another way, since the angle of cutting edge loading varies widely, the arcuate surface presents a range of normal vectors to the resultant force vector loading the cutting edge so that at least one of the normal vectors will, at any given time and under any anticipated resultant loading angle, be parallel and in opposition to the loading. Thus, at the area of greatest stress experienced at the interface, the superabrasive material and adjacent substrate material will be in compression, and the interface surface will lie substantially transverse to the force vector, beneficially dispersing the associated stresses and avoiding any shear stresses.